This invention concerns a method and apparatus for reducing the amounts of gaseous pollutants, particularly acid pollutants, contained in gaseous emissions such as the smoke produced in burning various fuels or in incinerating wastes, including household garbage.
Smoke resulting from the burning of many fuels containing quantities of sulfur carries with it a certain amount of sulfur dioxide or sulfurous acid anhydride (SO2).
Smoke from the incineration of wastes containing chlorine, chiefly in the form of chlorous plastic materials (especially polyvinyl chloride (PVC)) will contain hydrogen chloride gas (HCl) in quantities that vary with the origin of the wastes. In household garbage, for example, the average HCl content is close to 1 g/Nm3 under standard conditions of evaluation (7 percent CO2 or 11 percent O2), for which maximum discharge amounts have been established. For such wastes, the SO2 content of the smoke may be, by contrast, relatively low (on the order of several hundred milligrams per Nm3), whereas it may reach several grams with certain fuels.
Depending on the circumstances of combustion, regardless of what is being burned, smoke and fumes will also contain nitrous oxides (NOx) and often traces of other acid pollutants such as hydrofluoric acid (HF).
In order to protect the environment and to comply with current regulations, smoke and fumes must be treated so as to lower the amount of acid pollutants they contain.
Because these acid pollutants are water soluble, many antipollution processes consist of washing or scrubbing the smoke or fumes with water.
Other known processes recommend injecting one or more basic reagents into the smoke, using one of the three following procedures:
1.dry method: reagents are injected in powder form, or the gases pass over beds of the reagents, with the beds generally being stationary;
2. semi-dry method: reagents are injected in the form of a solution that is sufficiently concentrated to allow the reaction products to be collected in powder form;
3. wet method: reagents are injected in the form of a solution, and the reaction products are also removed in the form of a solution.
Injecting neutralizing substances such as lime quickly results in fouling due to deposits of the products formed. This in turn requires periodic or cyclical cleaning by chemical or mechanical means.
Preferentially, the reagents are selected from among the carbonates or, better, from the alkaline or alkaline-earth hydroxides, usually unhydrated lime (CaO) or hydrated lime (Ca(OH)2).
Whatever reagent is used, a circuit for injecting it into the smoke must be provided, as well as another circuit for removing the reaction products: chlorides, sulfates, alkaline or alkaline-earth, powder or solution. Operating this part of the installation represents a sizable segment of the total cost of treating emissions. Installing it entails a considerable increase in investment.
Given the kinetics of reaction, it is necessary to use reagents in quantities two or three times greater than the stoichiometric quantity in dry-method processes, and at least the stoichiometric quantity in the semi-dry or wet-method processes.
The process of the invention makes it possible to reduce the amount of water-soluble gaseous acid pollutants contained in smoke or fumes resulting from combustion to a level that will be below regulatory maximums without injecting the smoke or fumes with water or a reagent in solution or powder form.